The present invention relates to a dry-type developing device for electrophotography, and more particularly to such a dry-type developing device for increasing or decreasing the amount of a developer supplied for developing a latent image dependent on the amount thereof consumed.
As disclosed in U.S. Pat. No. 4,267,248, a conventional dry-type developing device has a developing roll disposed in confronting relation to an electrostatic latent image carrier. The developing roll is composed of a hollow nonmagnetic sleeve and a permanent magnet, and is rotatable about its own axis. The permanent magnet is disposed in the hollow nonmagnetic sleeve and has alternately positioned S and N poles. A developer tank placed adjacent to the developing roll contains a developer therein. The developer tank has a cutout through which a portion of the developing roll enters into the developer tank to define a first opening (developer supply port) and a second opening (developer retrieval port) between the developing roll and the developer tank.
In operation, the developer in the tank is magnetically attracted to the peripheral surface of the sleeve and discharged from the tank through the first opening toward a developing region in response to rotation of the sleeve and the permanent magnet. In the developing region, part of the developer supplied from the tank is attached to an electrostatic latent image on the electrostatic latent image carrier for developing the latent image. The remaining developer which has not been attached to the electrostatic latent image is retrieved through the second opening into the developer tank.
The amount of the developer consumed in the developing region is largely dependent on the density of the latent image on the electrostatic latent image carrier, i.e., the printing density. More specifically, when an image of a high printing density is developed, the developer is temporarily consumed in a large amount, and thereafter an amount of the developer necessary for proper image developer cannot be supplied, resulting in a reduced printing density.
The conventional developing device of the type described therefore is required to have various means for keeping a constant printing density to ensure a good image quality. For example, U.S. Pat. No. 4,082,445 discloses a system for comparing, with a reference value, the difference between the intensity of light reflected from an undeveloped area on a photoconductive layer and the intensity of light reflected from a developed test image, and for supplying a developer to a developing device to increasing the printing density if the density of the test image is insufficient. Another means includes a density sensor for automatically detecting the density of characters printed on a sheet of print paper to control the amount of light radiated on an electrostatic latent image carrier or to control the voltage of a primary charging electrode for charging the electrostatic latent image carrier, dependent on the detected density level.
With the developing device incorporating the above means, however, the density of characters or images is detected while the print paper is running. The detected density if therefore unstable in level, and the printing density cannot be uniformized at a desired level. In addition, the developing device has been costly to manufacture.